1. Field of the Invention
The present invention relates to a zoom lens apparatus, and more particularly to a zoom lens apparatus to be applied to monitoring cameras for use in daytime and nighttime.
2. Description of the Related Art
In a mechanical compensation type zoom lens apparatus applied to monitoring cameras, any focus blurring due to the shifting of a variable-magnification lens is compensated for by shifting a correcting lens interlocked with the variable-magnification lens.
However, as the shifting of the correcting lens in a conventional zoom lens apparatus is regulated so as to compensate for any focus blurring when in use under visible light, there is a problem that, when the apparatus is used under infrared light at nighttime or whenever else, the shot goes out of focus under the influence of chromatic aberrations due to a difference in wavelength of subject light.
In view of this problem, in a zoom lens apparatus according to Japanese Patent Application Publication No. 8-94911, one of the inner wall faces of a cam groove for regulating the shifting of the correcting lens is shaped for visible light, and the other for infrared light, and any focus blurring due to a difference in wavelength of subject light is compensated for by switching the face that comes into contact with a cam pin according to the wavelength ranges to be used.
However, since the zoom lens apparatus according to Japanese Patent Application Publication No. 8-94911 permits only two alternative peak wavelength ranges to choose from, its versatility is limited, resulting in a disadvantage that accurate focusing compensation according to the shooting environment is impossible.
An object of the present invention, attempted in view of this circumstance, is to provide a zoom lens apparatus capable of accurately compensating for any focus blurring due to variations in wavelength of subject light.
In order to attain the above-described object, the present invention is directed to a zoom lens apparatus, comprising: a variable-magnification lens which is held by a first lens frame and movable along an optical axis in a lens barrel; and a correcting lens which is held by a second lens frame and movable along the optical axis in the lens barrel, wherein: the variable-magnification lens is shifted so as to vary a focal distance of the zoom lens, and the correcting lens is shifted so as to compensate for any focus blurring due to the shifting of the variable-magnification lens; at least one of the first and second lens frames comprises: an inner frame which holds the lens; an outer frame which holds the inner frame to be shiftable back and forth along the optical axis; and a drive device which shifts the inner frame back and forth relative to the outer frame; and any focus blurring due to variations in wavelength of subject light is compensated for by causing the drive device to shift the inner frame relative to the outer frame and thereby shifting at least one of the variable-magnification lens and the correcting lens.
Preferably, the inner frame and the outer frame have a threaded outer surface and a threaded inner surface, respectively, engaged with each other; and the drive device is arranged on the outer frame and rotates the inner frame relative to the outer frame so as to shift the inner frame back and forth along the optical axis.
Preferably, the zoom lens apparatus further comprises: a selector device which selects a wavelength of subject light; a memory device in which information on a position on the inner frame relative to the outer frame is stored for each wavelength of subject light selectable by the selector device; and a control device which so controls the drive device as to shift the inner frame to the position matching the wavelength of subject light selected by the selector device according to the information stored in the memory device.
According to the present invention, the lens frame for the variable-magnification lens or the correcting lens is configured of an inner frame and an outer frame, and the variable-magnification lens or the correcting lens is shifted along the optical axis as the drive device shifts the inner frame relative to the outer frame. This makes it possible to compensate for any focus blurring due to wavelength variations steplessly.